1. Field of the Invention
The present invention relates to devices for handling paper webs which support a web on a cushion of air in general, and to devices for supporting tissue paper webs in particular.
2. Description of the Prior Art
Lightweight webs of paper with high absorbency are created by pressing or thru-air drying the wet web and placing it onto a large diameter dryer roll and scraping the dried web off the roll with a doctor blade. The scraping of the dryer roll results in creping of the web. Creping shortens the machine direction length of the web while at the same time making the web thicker and also breaks some of the fiber bonds within the paper. These changes in the web make it better able to absorb water. Creped paper webs are fabricated into products such as facial tissues, paper napkins, paper towel, and toilet paper.
The creping action of the doctor blade results in a web which is easily stretched in the machine direction and to a lesser extent in the cross machine direction. However, stretching of the tissue web is undesirable because it reduces the absorbency of the web. For this reason it is important to avoid excessive pulling or tensioning of the web such as can result when the web is turned or directed around one or more rollers, foils or turning bars. One solution is to utilize turning bars or foils with air jets which support the web on a cushion of air. This type of air support is often used with coated paper when handling the paper before the coating has dried completely.
Tissue paper, because of its ability to stretch, can easily become wrinkled as the unconstrained paper moves over these supports, leading to wrinkles in the finished product. Wrinkles lower the product's functionality by reducing absorbency and detracting from the appearance of the products formed from tissue paper.
U.S. Pat. No. 4,342,313 to Reba discloses a turning bar where more air is supplied to the center of the web by varying the width of a slit along the length of the turning bar to establish a desired air flow profile. Reba suggests that by making the slit wider in the center than at the edges, more air cushioning in the middle of the web is obtained which creates cross machine direction spreading and improved guiding of the web.
Long et al. U.S. Pat. No. 5,209,387 describes an apparatus for conveying photographic film where the web of film is controlled with reasonable precision by adjusting the gas pressure or changing the size or angle of the gas apertures.
What is needed is a simple turning bar about which a web of tissue paper can be turned without wrinkling or significant stretching.